Development of reliable biomarkers for both dopaminergic and non-dopaminergic manifestations of Parkinson disease (PD) and related disorders may dramatically accelerate research on PD etiology, pathophysiology, and therapeutics. Biomarkers are broadly defined as characteristics that are objectively measured and evaluated as indicators of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention. Specific biomarkers may be useful at the onset of neurodegeneration, the onset of disease, and/or to mark disease progression.